Silver Complex-Modified Tetravanadium-Substituted Polyoxomolybdates Containing Ordered Tunnels for Supercapacitors and Electrocatalytic CO2 Reduction

Abstract

Assembly of rigid ligands and metal-organic units into 4-V-substituted cap-type Keggin arsenomolybdate to generate two hybrid derivatives, (Hbipy)2[AsⅢ2AsMoVMoⅥ7V4O40]·5H2O (AMV-1), (bipy)0.5[{Ag2(bipy)4}{(VO) (AsⅢAsMo8V4O40)}]·H2O (AMV-2) (bipy = 4,4’-Bipyridyl) via hydrothermal method. AMV-1 is a 3,6-connected porous supramolecular network modified by bipy ligands with a {4;62}2{42;68;85} topology. In AMV-2, {Ag2(bipy)4} and {Ag(bipy)2} are alternately connected by sharing bipy to yield Ag-bipy metal-organic layer. Keggin clusters are suspended between adjacent layers, forming a unique polyoxometalate-based organic-inorganic hybrid network with ordered tunnels. Compared to AMV-1, AMV-2 demonstrates superior capacitive performance. An asymmetric supercapacitor using AMV-2-CPE as the negative electrode showed a specific capacitance of 171.8 F g-1 at 2 A g-1, with a high energy density (37.2 Wh kg-1) and a power density (1249.2 W kg-1). After 10000 charge-discharge cycles, 92.8% of the initial capacitance is retained. This excellent electrochemical performance arises from the synergistic effect between the porous host-guest architecture induced by vanadium substitution in the Keggin cluster and the conductive Ag-bipy framework. Additionally, the AMV-2-CPE electrode achieved a CO2 reduction current density of 25 mA cm-2 at -0.8 V vs. RHE with 90.9% Faraday efficiency for CO production. DFT calculations indicate the {Ag2(bipy)4} fragments create localized positive electrostatic potential around Ag sites, enabling stable CO2 adsorption. Synergistic catalysis between active Ag sites and the POM significantly lowers the free energy barrier of the rate-determining step.